Multi-wavelength programmable laser processing mechanisms and apparatus utilizing vaporization detection

ABSTRACT

A desired design for electronic structures is converted into a graphic design format and sorted into a pseudo-raster format corresponding to scan lines. A laser or other machining beam is controlled by a separate tracking beam utilizing a mid-objective scanning system. The firing frequency of the machining beam is determined by the position of the tracking beam on a detector, as compared to the scan line data. Accuracy is verified by detection of plume or spectra generated during machining. Alignment of the machining and tracking beams is by interferometric methods. The system improves optical performance parameters of telecentricity, angle of scanned beam line, location of line in which the scanned line resides, astigmatism and field curvature.

This is a continuation in part of U.S. application Ser. No. 02/430,480filed Apr. 28, 1995 U.S. Pat. No. 5,620,618.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method and apparatus formachining or otherwise processing a workpiece such as an electronicsubstrate with the aid of a high energy beam and a tracking beam whichcorrects astigmatism, bow distortion and field curvature, and moreparticularly to a laser processing mechanism which incorporates anastigmatically corrected catadioptric laser scanner.

2. Description of Related Art

In the current manufacture of multilayer ceramic (MLC) substrates forintegrated circuit semiconductor package structures, a plurality ofgreen ceramic sheets is formed by a doctor blading a slurry containing aresin binder, a particulate ceramic material, solvents, and aplasticizer, drying the doctor bladed sheet and cutting it intoappropriate sized sheets. Via holes are then mechanically punched forforming electrical interconnections through the sheet. Electricallyconductive paste is deposited in the holes, and in appropriate patternson the surface of the sheets, the sheets stacked and laminated, and theassembly subsequently fired at an appropriate sintering temperature.Punching the via holes in ceramic sheets presents formidable engineeringproblems in view of the small size, high density, and the complexpatterns of the via holes. Apparatus used to perform these operationsare described in IBM Technical Disclosure Bulletin (TDB) Vol. 13, No. 9,Feb. 13, 1971 P. 2536; IBM TDB Vol.16, No. 12, May 1974 P. 3933; IBM TDBVol. 20. No. 4, September 1977, P.1379; and U.S. Pat. Nos. 4,425,829;3,730,039; and

4,821,614, the disclosures of which are incorporated by referenceherein.

The mechanical punching technology currently used to manufacture MLCsubstrates has several limitations. The aspect ratio of a hole shouldtheoretically be no less than one, that is the diameter should not beless than the thickness of the sheet to be punched. As theminiaturization of electronic devices continues, the requirement thatsmaller via holes be used increases. A certain minimum sheet thicknessis necessary, however, for the mechanical integrity of the structure.

In addition to requiring smaller diameter holes, future electronicsdevices will require that the holes be spaced closer together. Use of amechanical punch at these geometries causes greatly increased embossingof the green sheet, which can greatly distort the via pattern.

High energy beams, including lasers, have been used to machine a varietyof workpieces and this activity has been widely reported in theliterature. Examples of using lasers to drill holes in electronicsubstrates are described in U.S. Pat. No. 4,544,442 and in U.S. Pat. No.4,789,770. An apparatus combining laser machining with a mechanicalpunch is shown in U.S. Pat. No. 4,201,905.

A rotary metal removing operation such as reaming is not adaptable toextremely small hole diameters of high production rates, however. Thesetypes (small hole diameter or high production rates) of applicationsrequire a pulsed, high power laser which, if not accurately positioned,can improperly machine or ruin the workpiece. Because of this pulsing,it is not possible to use the beam to determine the machining position,since any possible damage will have already occurred by the time theposition is determined. A low energy continuous laser, such as a HeNelaser, collinear with the high energy laser has been used to determinethe position of the high energy laser. A control system which allowsimplementation of this idea is described in U.S. Pat. No. 3,902,036. Useof a similar control system for ophthalmic applications is described inU.S. Pat. No. 4,520,816.

A particularly useful apparatus and method for accurately and rapidlypositioning and machining a workpiece comprising an electronic substrateis disclosed in U.S. Pat. No. 5,168,454, assigned to the assignee ofthis application. A low power HeNe laser is joined colinearly to theoptical path of a high power pulsed Nd:YAG laser, and the collinear beamthen scans one axis of the workpiece. The low power beam is partiallysplit off to a location-determining device before final deflection ofthe beams to the workpiece. Deflection in the second axis is achieved bylinearly moving the workpiece so that the beam will impinge upon thedesired location of the workpiece.

Given the requirement for smaller and more closely spaced features inMLC substrates, a need exists for an apparatus and a method which useadvanced technology to manufacture substrates with the requiredgeometries. This apparatus and method must be able to accurately andrapidly machine features in these substrates in order to provide thenecessary feature geometries and yet remain competitive with existingmechanical devices such as the multiple-punch apparatus described inU.S. Pat. No. 4,425,829.

A need also exists for an apparatus and method for verifying that thefeatures machined in the workpiece are correctly located. Although aseparate apparatus for performing this function has been previouslydescribed, for example in U.S. Pat. No. 4,555,798, a need exists for anapparatus and method which are capable of being integrated into themachining apparatus and method. Alignment of the optics in scanningsystems are also critical, and many prior art systems are not practicalfor use in integrated machining systems.

Scanning systems using lasers are used in many applications. Laserscanners are part of a growing multi-billion dollar industry. Forexample, laser light could be scanned to drill holes in a semiconductorsubstrate to create micro-circuitry, or it could be used to scribealpha-numerics on a part or be used to read bar codes or used in laserprinters.

The laser scanning systems can be basically classified into three types:Objective Scanners, Pre-Objective Scanners, and Post-Objective Scanners.

Objective Scanners 19, as shown in FIG. 1A and 1B, are the types ofscanners which use a simple lens 10, to focus a beam of light 25, suchas a laser beam 25, onto a workpiece or a part 12. The focused laserbeam 25, is then scanned over the part 12, by moving the part 12, asshown in FIG. 1B. A major advantage of Objective Scanners is that theoptics are less complex. Major disadvantages of Objective Scanners aretheir slower scan speeds and requirements of complex strategies to movethe lens or the part.

Pre-Objective Scanners 29, as shown in FIG. 2, are the types of scannersthat have a moving, mirrored surface 22, typically a galvanometer or arotating mirrored polygon, which reflects the laser beam 25, into a lens20. The lens 20, then focuses the laser beam 25, onto a part 12, atlocation 23. When the mirrored surface changes its angle, mirroredsurface 22', directs the laser beam 25, at a different angle andposition into the lens 20. The lens 20, then focuses the beam 25, toanother point 27, on the part 12, as can be clearly seen in FIG. 2.Generally, the lenses 20, in a Pre-Objective scanning system 29, arecomplex and expensive. Major advantages of Pre-Objective Scanners 29,are their high scan speeds and their ability to have a flat-field image.However, the major disadvantages of Pre-Objective Scanners 29, are thatthe lenses are very complex, the lenses are not telecentric (atelecentric lens allows the center for the scanned beam of light toimpinge the work surface orthogonally throughout the scan) unless thelens is very large, the system is complex, color correction is verydifficult and all these features make the system very expensive.

Post-Objective Scanners 39, as shown in FIGS. 3A (side view) and 3B (topview), are the types of scanners that have a moving mirrored surface 22,usually a galvanometer or a rotating mirrored polygon, after a focusingobjective lens 30. The light or laser beam 25, after passing through alens will also be referred to a light or laser beam 125. The laser beam25, first passes through the lens 30, which starts to bring the laserbeam 125, to a focus. The laser beam 125, is interrupted and reflectedby the galvanometer or the mirrored surface 22, to focus on the surfaceof the part 12, at point 35. When the scanning mechanism changes itsangle it redirects the focus of the beam 125, as illustrated in FIG. 3A,to either spot 33 or 37, on an imaginary curve or arc 31. The beam 125,is perfectly focused on the arc 31, but it is out of focus at points orspots 34 and 38, on the flat surface 32, of the workpiece or part 12.For the Post-Objective Scanner 39, in FIG. 3A, the laser beam 125, isfolded by the galvanometer 22, 90 degrees in the X-Y plane, so that thelaser beam focuses at point 35. When the galvanometer 22, scans the beam125, it rotates about an axis 135, which lies in the center of thefocusing beam 125, on the surface of the galvanometer 22, and in thedirection of the Z-axis. The galvanometer 22, moves so that itintersects the laser beam 125, at an angle which is not 90 degrees inthe X-Y plane. This causes the focused beam to scan along the arc 31.FIG. 3B, shows this Post-Objective Scanner 39, from a view that looksdown on the scanned arc 31. Although the scanned arc 31, is out of focusat the edge of the scan it travels a straight path in the X-Z plane.Generally, the lenses 30, in a Post-Objective scanning system 39, aresimple and inexpensive. Major advantages of Post-Objective scanners aretheir scan speeds, simplicity of the object lens, color correctabilityand their ability to be designed for more wavelengths. Majordisadvantages include the fact that the image field is out of focus atthe edges of the scanned field or not in focus throughout the scanneddistance.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide an apparatus anda method that will provide an astigmatically corrected catadioptriclaser scanner.

A further object of the invention is to provide a means for minimizingbow distortion, astigmatism, and field curvature resulting from aPost-Objective Scanner system.

It is yet another object of the present invention to provide a scannersystem that uses very inexpensive lenses and mirrors.

It is a further object of this invention to provide a beam that istelecentric, i.e., the center of the focusing beam is perpendicular tothe part at all points along the scan.

It is still yet another object of the invention to provide an opticalsystem which is inexpensively adaptable to different wavelengths oflight.

It is a further object of this invention to provide an optical scanningsystem in which the length of the scanned line is directly proportionalto the angular change of the moving mirrored surface. This is the resultthat is sought for a so-called F-theta lens.

It is another object of the present invention to provide a method andapparatus for producing and machining microelectronic components andsubstrates with densely packed small feature connections.

It is a further object of the present invention to provide a method andapparatus for producing and machining microelectronic components andsubstrates which permits high throughput (productivity) and low defectrates.

It is yet another object of the present invention to provide a methodand apparatus for producing and machining microelectronic components andsubstrates which may be utilized in "just-in-time" systems and isadaptable to meet future production and growth requirements.

It is still another object of the present invention to provide animproved method and apparatus for aligning, adjusting, tracking,controlling, and verifying machining in a system utilizing (laser)machining beams.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects are achieved in the present invention whichincorporates an improvement of the Post-Objective Scanner type and it isa fourth type of a scanning system for laser or other beam sources whichwill now be referred to as a Mid-Objective Scanner.

The present invention utilizes a novel method and an apparatus for a newastigmatically corrected catadioptric laser scanner, which method may beincorporated into a machining and tracking system.

Therefore, in one aspect that invention comprises a method forcorrecting astigmatism, bow distortion and field curvature comprising:

(a) at least one lens tilted at an angle alpha for focusing at least oneincoming light beam onto at least one workpiece,

(b) at least one scanning mechanism for interrupting and scanning the atleast one incoming light beam,

(c) the at least one scanning mechanism further directing the at leastone incoming light beam onto at least a portion of the reflectivesurface of a concave cylindrical mirror, and

(d) wherein the concave cylindrical mirror directing the at least oneincoming light beam onto the workpiece, such that the at least oneincoming light beam is focused at the workpiece.

In another aspect this invention comprises an apparatus for correctingastigmatism, bow distortion and field curvature comprising:

(a) at least one means for tilting at least one lens alpha degrees tofocus at least one incoming light beam onto at least one workpiece,

(b) a scanning mechanism located between the at least one tilted lensand the at least one workpiece, wherein the scanning mechanism has atleast one means for directing the at least one incoming light beam ontoat least a portion of the reflective surface of a concave cylindricalmirror, and

(c) a means in the concave cylindrical mirror for directing the at leastone incoming light beam onto a workpiece, such that the at least oneincoming light beam is focused at the workpiece.

In yet another aspect this invention comprises an apparatus forcorrecting astigmatism, bow distortion and field curvature comprising:

(a) at least one toric lens to focus at least one incoming light beamonto at least one workpiece,

(b) a scanning mechanism located between the at least one toric lens andthe at least one workpiece, wherein the scanning mechanism has at leastone means for directing the at least one incoming light beam onto atleast a portion of the reflective surface of a concave cylindricalmirror, and

(c) a means in the concave cylindrical mirror for directing the at leastone incoming light beam onto a workpiece, such that the at least oneincoming light beam is focused at the workpiece.

In still yet another aspect this invention comprises a method forcorrecting astigmatism, bow distortion and field curvature comprising:

(a) at least one toric lens for focusing at least one incoming lightbeam onto at least one workpiece,

(b) at least one scanning mechanism for interrupting and scanning the atleast one incoming light beam,

(c) the at least one scanning mechanism further directing the at leastone incoming light beam onto at least a portion of the reflectivesurface of a concave cylindrical mirror, and

(d) wherein the concave cylindrical mirror directing the at least oneincoming light beam onto the workpiece, such that the at least oneincoming light beam is focused at the workpiece.

The Mid-Objective Scanner may be incorporated into a simplified, lowcost optical delivery system for a high-powered machining (e.g.,cutting) beam and a low powered tracking beam which utilizes uniquealignment techniques and computer optical positioning control methods toprovide a material processing instrument that achieves significantresolution (small feature size) and positional accuracy. The inventionachieves a significant reduction in the overall cost of processingelectronic components and substrates by: (a) reducing complexity andcost of direct hardware, (b) allowing an application to be programmableand therefore adaptable to product set changes in a "just-in-time"system by an integrated computer optical control system, and (c)allowing for adaptability for a range of (laser) beam sources by use ofa simplified optical delivery path. The present invention removes theneed for additional time and effort to produce and switch complex fixedmasks for components to be machined. A single investment in capitalmachinery can be utilized for different material and workpiece sets andmay allow for growth in future manufacturing requirements.

Therefore, in a further aspect, this invention provides a machiningsystem comprising means for generating a machining beam; and means forgenerating a tracking beam, the tracking beam being separate from themachining beam. There is also provided a means for scanning themachining and tracking beams along an axis, the scanning means beingmovable and having at least one surface for reflecting said machiningbeam and for reflecting said tracking beam. Preferably the scanningmeans has a first surface for reflecting the machining beam and a secondsurface for reflecting the tracking beam, the first and second surfacesof the scanning means being fixed relative to each other. Alternatively,the tracking beam is reflected by the same surface of the scanning meansas the machining beam, but at a different angle of incidence. Aworkpiece holder secures a workpiece to receive the machining beam afterreflection from the first surface of the scanning means. A detectorreceives the tracking beam after reflection from the second surface ofthe scanning means. A feedback control means then determines theposition of the reflected tracking beam relative to the detector andcontrols and changes the firing rate of the machining beam as themachining beam is being scanned by determining the position of themachining beam relative to the workpiece based upon the position of thereflected tracking beam relative to the detector.

The machining system preferably utilizes the aforedescribed method andapparatus for correcting astigmatism, bow distortion and fieldcurvature, and includes at least two concave cylindrical mirrors havingcurved reflective surfaces. The curvatures of the reflective surfaces ofthe concave cylindrical mirrors are substantially identical. One concavecylindrical mirror is positioned to interrupt the reflected machiningbeam and at least partially focus the machining beam at the workpiece.Another concave cylindrical mirror is positioned to interrupt thereflected tracking beam and at least partially focus the tracking beamon a grating before the detector. The scanning apparatus is preferablybidirectional, but may also be unidirectional.

In yet another aspect, the present invention relates to a method ofcontrolling a machining system comprising the steps of:

(a) generating a machining beam having a variable firing rate;

(b) generating a tracking beam, the tracking beam being separate fromthe machining beam;

(c) providing means for scanning the machining and tracking beams alongan axis, the scanning means being movable and having at least onesurface for reflecting said machining beam and for reflecting saidtracking beam;

(d) simultaneously scanning the machining and tracking beams with thescanning means;

(e) securing a workpiece to receive the machining beam after reflectionfrom the first surface of the scanning means;

(f) detecting the tracking beam after reflection from the second surfaceof the scanning means;

(g) determining the position of the reflected tracking beam; and

(h) controlling the firing rate of the machining beam as the machiningbeam is being scanned by determining the position of the machining beamrelative to the workpiece based upon the detected position of thereflected tracking beam.

Preferably, in this aspect, the scanning means has a first surface forreflecting the machining beam and a second surface for reflecting thetracking beam, with the first and second surfaces being fixed relativeto each other. Thus, in step (d) the machining beam is scanned by thefirst surface of the scanning means and the tracking beam is scanned bythe second surface of the scanning means.

In another aspect, the present invention provides a method ofcontrolling a machining system comprising the steps of:

(a) providing a desired design for machining a workpiece;

(b) converting the design into a graphic design format comprisingpixels;

(c) sorting the pixels into a pseudo-raster format corresponding to atleast one scan line across a portion of the design for the workpiece;

(d) generating a machining beam having a variable firing rate;

(e) generating a tracking beam, the tracking beam being separate fromthe machining beam;

(f) providing means for scanning the machining and tracking beams alongan axis, the scanning means being movable and having at least onesurface for reflecting the machining beam and the tracking beam;

(g) securing a workpiece to receive the machining beam after reflectionfrom the scanning means;

(h) simultaneously scanning the machining and tracking beams with thescanning means, the machining beam being scanned along a scan axisacross a portion of the workpiece;

(i) detecting the tracking beam after reflection from the second surfaceof the scanning means;

(j) determining the position of the reflected tracking beam;

(k) comparing the position of the reflected tracking beam with the scanline across a portion of the workpiece design; and

(l) controlling the firing rate of the machining beam as the machiningbeam is being scanned by determining the position of the machining beamrelative to the workpiece based upon the workpiece design scan line.

In a further aspect, the present invention relates to a method ofaligning and adjusting a light beam-based machining system to improveoptical performance parameters of telecentricity, angle of scanned beamline, location of line in which the scanned line resides, astigmatism orfield curvature. The method comprises the steps of:

(a) providing means for generating a first, working beam;

(b) providing means for scanning the first beam along an axis, thescanning means being movable and having a surface for reflecting thebeam;

(c) providing a holder for securing a workpiece or detector to receivethe working beam after reflection from the surface of the scanningmeans;

(d) providing means for generating a second, alignment beam andtransmitting the alignment beam in place of the working beam over thesame path as the working beam;

(e) splitting the alignment beam into an object beam portion and areference beam portion prior to scanning the alignment beam;

(f) providing an optically flat mirror in the holder in place of theworkpiece or detector;

(g) scanning the object beam portion of the alignment beam across atleast one axis across the optically flat mirror in the holder;

(h) reflecting the object beam portion of the alignment beam back fromthe mirror in the holder;

(i) generating an interference pattern between the reference beamportion and the reflected object beam portion; and

(j) adjusting the scanning means based upon the interference pattern toimprove optical performance parameters of the machining system.

The scanning means preferably is that described previously and mayinclude a movable lens, a movable flat mirror, and a movable concavecylindrical mirror. The adjusting step may comprise moving some or allof the lens and flat and concave mirrors.

In yet another aspect, the invention relates to an apparatus forverifying machining operations comprising means for generating amachining beam and means for scanning the machining beam along an axis,as described previously. A workpiece holder is provided to secure aworkpiece to receive the machining beam after scanning by the scanningmeans. A detector is located proximate the workpiece holder fordetecting a plume generated by vaporization of material by the machiningbeam. Electronic control means verifies that the machining process endpoint has been achieved within the workpiece based upon the presence orabsence of a plume detected by the detector. The associated methodaspect comprises the steps of: (a) generating a machining beam; (b)scanning the machining beam along an axis over a workpiece; (c)detecting a plume generated by vaporization of material by the machiningbeam; and (d) verifying the progress of the machining process within theworkpiece by electronic control means based upon the presence or absenceof the plume. For example, a detected plume indicates further machiningis required since material is still being removed by the machining beam.

In a further aspect, the present invention provides an apparatus forverifying machining operations on a workpiece comprising at least twodifferent materials having different identifying spectra comprisingmeans for generating a machining beam and means for scanning themachining beam along an axis. A workpiece holder is provided forsecuring the workpiece to receive the machining beam after scanning bythe scanning means. A spectrometer is located proximate the workpieceholder for detecting identifying spectra generated by vaporization ofdifferent materials in the workpiece by the machining beam. Electroniccontrol means verifies that the proper material interface has beenreached (process end point) within the workpiece based upon theidentification of spectra of the different materials in the workpiecedetected by the spectrometer. The associated method aspect comprises thesteps of: (a) generating a machining beam; (b) scanning the machiningbeam along an axis; (c) detecting identifying spectra generated byvaporization of different materials in said workpiece by the machiningbeam; and (d) verifying the position of the machining beam relative tothe workpiece based upon the identification of spectra of the differentmaterials in the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIGS. 1A and 1B illustrate two typical prior art Objective Scanners.

FIG. 2 illustrates a typical prior art Pre-Objective Scanner.

FIGS. 3A and 3B illustrate the side view and top view respectively of atypical prior art Post-Objective Scanner.

FIG. 4 illustrates a method to correct the field curvature of the priorart Post-Objective Scanner as shown in FIGS. 3A and 3B.

FIG. 5A illustrates field curvature and bow distortion induced bytypical prior art Post-Objective Scanners.

FIG. 5B illustrates the top view of the Post-Objective scanning systemof FIG. 5A, in the X-Z plane.

FIG. 6 illustrates astigmatism induced by a lens.

FIG. 7 illustrates a preferred embodiment of the invention.

FIG. 8 illustrates another embodiment of the invention using a toriclens.

FIG. 9 illustrates a preferred embodiment of the machining and trackingsystem of the present invention.

FIG. 10 illustrates a side view of a galvanometer used in the preferredmachining and tracking system of FIG. 9.

FIG. 11 illustrates a perspective view of a rotating polygonal mirrorwhich may be used in the machining and tracking system of FIG. 9.

FIG. 12 illustrates a front elevational view of the grating of thephotodetector used in the preferred machining and tracking system ofFIG. 9.

FIG. 13 is a block diagram illustrating the control system used in thepreferred machining and tracking system of FIG. 9.

FIG. 14 illustrates a side elevational view of a workpiece, workpieceholder and verifying apparatus useful in the preferred machining andtracking system of FIG. 9.

FIG. 15 illustrates the interferometric alignment system used in thepreferred machining and tracking system of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-15 of the drawings in whichlike numerals refer to like features of the invention. Features of tileinvention are not necessarily shown to scale in the drawings.

MID-OBJECTIVE SCANNER

This invention relates more particularly to an astigmatically correctedcatadioptric laser scanner or a Mid-Objective Laser Scanner system. Acatadioptric system is a system that uses both reflection and refractionto achieve its focal power. While the relative powers of the lenses andmirrors vary from system to system, the use of the reflective surfacesto achieve most of the power, in combination with refractive surfaces oflittle or zero power, produces an image that has improved aberrationalcharacteristics.

A lens as used herein means a single lens element or a plurality of lenselements. This invention may also utilize a lens that is achromatic sothat multiple colors of light or multiple wavelengths of light could beused in the inventive Mid-Objective scanning system.

Referring back to FIGS. 3A and 3B, when a Post-Objective Scanner 39,scans the focused laser spot 35, on an imaginary line or curve 31, itswings the focus point or spot 35, in an arc or curve 31, which hasradius of curvature, which radius of curvature is exactly the samedistance from the center of the galvanometer 22, to the focus point 35,as shown in FIGS. 3A and 3B. This creates a deviation from a flat imageplane which is called field curvature 36. Field curvature 36, is thedistance between the arc 31, and the flat surface 32, of the part 12. Ascan be clearly seen in FIGS. 3A and 3B, the arc 31, is completelyfocused at point 35, however when the beam 125, is scanned in eitherdirection to spot 33 or 37, beam 125, goes out of focus on the flatsurface 32, of the part 12, as can be seen at point 34 and 38,respectively, on the part 12. A perfect scan would be if the focusedspot 35 followed a straight scan line oil the flat surface 32 on thepart 12 to properly scan the part 12.

This field curvature 36, can be corrected using a Post-Objective Scanner49, as shown in FIG. 4, by reflecting the scanned beam 125, off of acurved mirror 42, having a correct radius of curvature 48, as shown inFIG. 4. This creates a straight scan line or a flat image field 41, andas one can clearly see that focused points 43, 45 and 47, all arefocusing on the imaginary image plane or field 41. However, this flatimage plane 41, is not physically accessible because tile part 12, mustlie between tile laser source and the curved mirror 42, and for mostapplications the laser beam 125, cannot generally pass through the part12.

As shown in FIGS. 5A (side view) and 5B (top view), when aPost-Objective Scanner 59, having a moving mirrored surface 50, 50',redirects the laser beam 125, at an angle of 90 degrees in the X-Yplane, but then scans the focused spot by rotating the galvanometer 50,about an axis 54. The path that the scanned spot takes 51, is not onlycurved out of focus from the flat part or workpiece 12, but is alsocurved away from the straight line 52. This deviation of the actual scanpath from a straight line is called bow distortion. This bow distortioncan be clearly seen in FIG. 5B, which is a view of the scanning system59, as seen by looking directly at the work surface 12, through thegalvanometer 50. The locus of focused spots not only lies on a curvedsurface in the X-Y plane (i.e., producing field curvature) the scannedline is curved in the X-Z plane (i.e., producing bow distortion). Itshould be noted that this bow distortion is not created when thegalvanometer scans the focused spot or point in the X-Y plane as shownin FIGS. 3A and 3B.

One way to correct this bow distortion has been discussed in a paper byVictor J. Doherty of Eidolon Corporation, entitled "Correction Schemefor a Post-Objective Optical Scanner", Society of Photo-OpticalInstrumentation Engineers, International Lens Design Conference, Vol.554, Pages 247-251 (1985). In his scheme he discusses the use of aspherical mirror similar to that of 42, shown in FIG. 4, but tilted at a45 degree angle to make the image plane accessible. However, thiscorrection adds an aberration to the image called astigmatism.

A typical example of astigmatism induced by a lens is illustrated inFIG. 6, where the lens aberration results in the tangential and sagittalimage planes being separated axially, or where one axis of the focusinglight beam comes to focus before the other axis. As shown in FIG. 6, inan optical system 60, having an object point 61, and optical axis 65,tangential fan of rays 62, come in focus at the tangential image orfocal line 64, while the sagittal fan of rays 66, come in focus at thesagittal image or focal line 68.

In the Eidolon correction scheme the correction of the two aberrationswas attempted by making the mirror toroidal, similar to that of mirror42, as shown in FIG. 5A, instead of spherical, i.e., the folding mirrorhas different spherical curvatures in each axis. It has been seen thatthe toroidal mirror does not correct all of the bow distortion in thesystem, and it also does not correct all of the astigmatism. The Eidoloncorrection scheme basically offers a compromise between the correctionof the two aberrations, i.e., one due to the bow distortion and theother due to astigmatism.

This invention provides a new method and apparatus to correct both theastigmatism and the bow distortion and also the field curvature.

The preferred embodiment of the invention is a Mid-Objective Scannersystem 79, as shown in FIG. 7, which uses a simple refractive objective70, which is tilted at an angle α relative to the optical axis, to beginto bring an incident laser beam 25, to focus. The beam 125, isinterrupted by a scanning mechanism 50, such as a galvanometer or arotating mirrored polygon, and is reflected 90 degrees by it in the X-Yplane. The galvanometer 50, scans the laser beam 125, by rotating aboutan axis 74.

Before the laser 125, comes to focus, the center of the laser beam inthe center of the scan is again folded 90 degrees by a concavecylindrical mirror 77, having a radius of curvature 78, in the sameplane that the galvanometer 50, folded the beam before, but it is foldedin such a way as to redirect the laser 125, in the same propagationdirection it had before it was folded by the galvanometer 50. Thecurvature of the concave cylindrical mirror 77, is typically tilted at a45 degree angle in the X-Y plane, and is of a radius of curvature 78,that corrects the field curvature in the image plane 75.

By folding the beam 125, again in this way, bow distortion is againadded to the system, but it is opposite to that of the bow distortionadded by the galvanometer 50. Thus, the bow distortion cancels itselfout and the image is free of it.

After the bow distortion has been corrected the only degradingaberration left in the system is astigmatism. This astigmatism is takenout of the image by adding the opposite amount of astigmatism in thefirst, simple, refractive objective 70. This is accomplished by tiltingthe lens 70, at an angle α, in the plane that the laser 125, is beingfolded (the X-Y plane) as shown more clearly in FIG. 7.

The amount or angle of the tilt α, is dependent upon several factors,such as, (a) the resolution of the system, measured by the numericalaperture, (b) the working distance from the reflective surface of theconcave cylindrical mirror 77, to the workpiece 12, (c) the distancebetween the moving mirrored surface 50, and the reflective surface ofthe concave cylindrical mirror 77, (d) the distance between the lens 70,and the moving mirrored surface 50, and (e) the focal length of the lensor the refractive objective 70, that is used, to name a few.

Similarly, the radius of curvature 78, for the concave cylindricalmirror 77, depends on factors, such as, (a) the distance between thelens 70, and the moving mirrored surface 50, (b) the distance betweenthe moving mirrored surface 50, and the reflective surface of theconcave cylindrical mirror 77, and (c) the focal length of the lens 70,to name a few.

If the lens 70, is titled about its nodal point, (a nodal point is animaginary point found for every lens), the focused image position willnot move as the lens 70, is tilted. This will allow the continuousobservation of the focused spot as the image quality is improved, muchlike watching a defocused image come in to focus. As the objective 70,is tilted, the laser beam 25, may be observed to identify the point atwhich the astigmatism is minimized.

FIG. 8, illustrates another embodiment of an inventive Mid-ObjectiveScanner 89, using a toric lens 80. A toric lens is a lens which has asurface having a maximum power in one meridian and a minimum power in aperpendicular meridian. A toric lens is typically used to correctastigmatism. The Mid-Objective Scanner 89, would be a little moreexpensive to build than the Mid-Objective Scanner 79, because of thetoric lens 80, however for some applications it might be desirable tohave such a lens. Toric surface 81, of the toric lens 80, should producethe astigmatism that needs to be added or compensated, in order toobtain a flat image or line 75, at the surface of the workpiece 12.

The at least one incoming beam of light 25 or 125, could be a laser beamor it could be a collimated beam of light or other electromagneticenergy.

This apparatus and process called the Mid-Objective Scanner enables theuse of a flat-field Post-Objective Scanner that is free from astigmatismand bow distortion. Additionally, the resulting scanned laser beam is amajor improvement over currently, available technology.

This invention removes bow distortion, astigmatism, and field curvature.It uses very inexpensive lenses and mirrors. It also provides atelecentric beam, i.e., a beam that when it is scanned the center of thebeam is perpendicular to the part at all points along the scan. Thisinvention also produces a scanned line in which the length of thescanned line is directly proportional to the scan angle of the movingsurface 50. This is a condition that is strived for in a scanning systemwhich is known as the F-theta condition.

One appropriate use or application for the Mid-Objective Scanner of thisinvention is to replace the optical scanning system in the MLDSCorrection Scheme, which is a correction system invented at IBMCorporation and a subject of U.S. Pat. Ser. No. 5,168,454 (LaPlante, etal.), the disclosure of which is incorporated herein by reference. MLDSis a Trade Mark of IBM Corporation, Armonk, N.Y., USA. Because theMid-Objective Scanner of this invention provides a telecentric lenssystem the holes that are drilled with the maskless laser drillingsystem (MLDS) of the above-mentioned U.S. Pat. No. 5,168,454, will allbe uniform throughout the scan, i.e., the drilled holes will not have anangle or tilted edge walls as the distance from the center of the scanchanges.

In a more preferred application in a material processing system, theMid-Objective Scanner of this invention is utilized in connection withboth a high energy cutting or machining beam and, unlike U.S. Pat. No.5,168,454, a separate low energy tracking or positioning beam.

Machining and Tracking System

Referring to the drawings in more detail and particularly referring toFIG. 9, there is shown the preferred machining apparatus according tothe present invention. The described apparatus and method can be used tomachine holes in a substrate and features other than holes, for example,slots, as described in U.S. Pat. No. 5,168,454. As used herein (andunless otherwise indicated), the term "machining" is intended toencompass all types of material processing or removal, either partial ofthrough the workpiece, including cutting, drilling, heating, heattreating, material deposition, exposing photosensitive emulsion, and thelike. The machining apparatus includes a means for generating a highenergy machining beam of pulsed or continuous nature and a means forgenerating a low energy positioning and verifying beam. For the purposesof the present invention, the source of the preferred high energy beamis a Nd:YAG laser 92 and the source of the preferred,low energy beam isHeNe laser 94. As for the case of exposing photosensitive emulsion, themachining beam could consist of a modulated low energy laser beam suchas that from a HeNe or Argon-Ion Laser. It should be understood, ofcourse, that this choice of lasers is not meant to be exhaustive asthere are other combinations of lasers and beams which will adequatelyfulfill the objects of the present invention, including multiple laserssuitably multiplexed.

The Nd:YAG laser 92 produces a beam 25 with a wavelength of 0.532microns and a diameter of approximately 1 mm. The beam 25 is pulsed,typically at a rate of 1000-2000 Hz, and has an average power in therange of approximately 3-10 W. After exiting the Nd:YAG laser 10 thebeam 25 may pass through an expander (not shown) which performs acollimating function to maintain the parallelism of the beam. The HeNelaser 94 produces a beam 96 with a wavelength of 0.5435 microns and adiameter of approximately 1 mm. The beam 96 is continuously powered witha power in the range of approximately 1-5 mw. The HeNe beam 96 also maypass through an expander (not shown) which performs a similar functionfor the HeNe beam as an expander would perform for the Nd:YAG beam.

The method and apparatus of delivery of the high energy laser beam 25 tothe workpiece 12 is the same system 79 as that previously described forthe Mid-Objective Scanner of this invention. Instead of the refractiveobjective lens 70 tiltable at angle α₁ shown in FIG. 9, one may employthe toric lens 80 shown in FIG. 8 or other suitable focusing device toproduce partially focused beam 125. Beam 125 strikes first surface 101of galvanometer 50 (FIG. 10) at angle β and is reflected off at angle βas beam 125'. After being reflected off concave cylindrical mirror 77,high energy beam 125" is focused and impinges workpiece 12 along imageplane axis 75.

Also provided is a means for translating workpiece 12 along a secondaxis. This translation means shown is translation table 116 which movesalong an axis 118 as indicated by the arrows in a direction which isorthogonal (i.e., perpendicular or normal) to the scan axis 75.

To be able to machine a variety of workpiece 12 materials, it isdesirable to be able to modify the wavelength of the machining beam 25.This may be done by methods known in the art. A wavelength of 1.06 μm issuitable for machining materials such as ceramic green sheets forelectronic substrates. A 0.266 μm machining beam is of particularimportance in the machining important insulating coating for electronicsubstrates. It can also be used to machine other dielectric materialssuch as Teflon, epoxy fiberglass, or other polymeric materials.

Next, the machining apparatus includes a method and apparatus 90 fortracking the high energy cutting or machining beam with a low energypositioning and verifying beam 96. The tracking apparatus employs aMid-Objective Scanner as previously described, but in a "mirror image"configuration. The tracking system portion of the invention uses acommon scanner with the machining portion of the invention. Trackingbeam 96 passes through refractive objective lens 98 (optionally, a toriclens or other focusing device) tiltable at angle α₂ and partiallyfocused beam 100 then strikes surface 102 of galvanometer 50 at angle β(FIG. 10). Surface 101 for high energy beam 125 is on the opposite sideof flat galvanometer 50 as surface 102 for low energy beam 100. Sincethe laser sources 92 and 94 are fixed, and surfaces 101 and 102 arefixed relative to each other, scanning movement of galvanometer 50 willproduce movement to tracking beam 100' identical to machining beam 125'.Preferably, laser sources 92 and 94 are initially aligned relative toeach other so that emitted beams 25 and 96, respectively, are collinearor coaxial. When the emitted beams 96 and 125 are collinear, or at leastparallel to each other, the axes of reflected beams 100' and 125' willbe parallel to each other.

Galvanometer 50 permits bidirectional scanning, i.e., scanning of a beamon a workpiece from one end of a scan axis to the other end, and thenretracing the scan axis from the second end back again to the first end.In this manner, machining, cutting or other processing may be performedwithout interruption while waiting for the scanner to relocate the beamback to the first end of the scan axis.

As shown in FIG. 11, another scanning means which may be employed inplace of galvanometer 50 is a rotating polygonal mirror 112. Whenrotating about axis 113 as indicated, the polygonal mirror acts as aunidirectional scanner, i.e., the incoming high energy beam 125 whichreflects off mirror segment 114 as beam 125' impinges the workpiece 12at one end of the scan axis and moves in one direction. When theincoming beam reaches the end of one mirror segment, the beam then againimpinges on the same end of the scan axis with the previous mirrorsegment and again moves in the same direction. Thus the beam does notretrace the original scan, as in scanning system utilizing thebidirectional galvanometer, 50 shown in FIG. 9. In the presentinvention, the bidirectional scanner is preferred. As shown in FIG. 11,incoming tracking beam 100 impinges an opposite mirror segment 114 andreflects off as beam 100'. When employed with a polygonal mirror havingan even number of sides, each of incoming beams 100, 125 and reflectedbeams 100', 125' will be parallel. However, there is no requirement touse an even number of sides in a rotating polygon or to use a two sidedgalvanometer mirror since the machining and tracking beams need not beopposite in direction. The tracking beam may even be reflected by thesame face of the scanner as the machining beam, but at a different angleof incidence.

After being reflected by surface 102 of galvanometer 50, and before itcomes into focus, the center of tracking beam 100' at the center of thescan is then folded 90 degrees by concave cylindrical mirror 104. Theradius of curvature 106 of concave mirror 104 is the same as that ofconcave cylindrical mirror 77. The tracking beam is folded in such a wayas to redirect beam 100' in the same propagation direction it had beforeit was folded by the galvanometer 50. As reflected by mirror 104,tracking beam 100" is corrected for bow distortion added by galvanometer50. The curvature of the concave cylindrical mirror 104 is typicallytilted at a 45 degree angle in the X-Z plane, and is of a radius ofcurvature 106 that also corrects the field curvature of focused beam100" on the image plane 110 on detector 108.

Photodetector 108 shown includes a flat glass plate or grating 120, asseen in FIG. 12. Chromium lines are shown as being applied to thesurface of the plate in two different uniform patterns--an upper pattern124 of thinner and more closely spaced vertical lines of, for example,0.015 mm width and 0.015 mm spacing, and a lower pattern 126 of thickerand more widely spaced vertical lines of, for example, 0.020 mm widthand 0.020 mm spacing. However, only one of the pattern sets is neededfor detection purposes. The tracking beam 100" passing through grating120 is detected as an on/off signal by a photoelectric device locatedbehind the grating, such as a charge-coupled device (CCD), and iscounted and analyzed, as will be discussed further below.

As discussed previously in connection with lens 70, to adjust andcorrect for astigmatism, lens 98 may be tilted about its nodal point toan angle α while observing the interferogram of the focused beam 125" onthe plane of detector 108 until the astigmatism is minimized.

To determine the location at which the high energy machining beam willimpinge upon the workpiece and control positioning of the high energymachining beam, position of the low energy tracking beam is determinedusing photodetector unit 108, which includes detector grating 120 andcomputer 122. Finally, as shown in FIG. 13, the machining apparatuscomprises an in-process means for verifying the machining process endpoint of the machining operation. This verifying means may include aphotometric, acoustic, mechanical or spectrographic detection system inconjunction with computer 122.

Tracking and Positioning Control System

The present invention is intended to be utilized with a maskless systemwhich relies upon stored machining information rather than a physicalmask placed over the workpiece to direct the machining beam to process(e.g., drill) the workpiece at desired locations.

In the present invention, scanning HeNe tracking beam 100" impinges uponthe detector grating 120 of photodetector 108, creating a signal whichis analyzed by an electronic circuit in a device such as a personalcomputer 122. By comparing the position of the positioning beam 100" ata given instant with a desired machining location on the workpiece 12,the computer can determine when to pulse the machining laser 92, as willbe explained in more detail hereafter.

Referring to FIG. 13, the method and apparatus which performs thisposition tracking and control function is described in more detail. Thescanning positioning beam 100" enters the front cavity of thephotodetector unit 108 and passes through a grating 120 before impingingupon a photoelectric device 128. This creates an approximatelysinusoidal signal 130 at a frequency of approximately 200 khz whichcorresponds to a sequence of discrete positions in the first axis 110across which the tracking beam 100" scans. The 200 khz signal isprocessed in a phase lock loop 132, creating a 2 Mhz signal 134. Thisprovides ten increments for each one incrementation, the grating givinga 10× accuracy improvement. This signal 134 is the input to an upcounter136, the output of which is compared by comparator 138 with a binarynumber, corresponding with the desired machining location, previouslyloaded into memory 140. When the upcounter 136 output matches the numberin memory 140 the comparator 138 will produce an output 142 whichtriggers the Nd:YAG machining laser, which will then fire, so that themachining beam 125" impinges upon the workpiece 12 in the properlocation. The output 142 of the comparator 138 is also an input toupcounter 144, which then sends an increment address signal to memory140, causing it to load the binary number corresponding to the nextdesired machining location.

Galvanometer 50 may scan at a constant frequency of, for example, 18 Hz.The machining Nd:YAG laser source 92 may have a variable firing rate of,for example 0-2000 Hz. Changing the timing of the machining laser firingrate, i.e., controlling when it will fire the next pulse, changes thelocation of impingement of the next beam pulse along scan axis 75 onworkpiece 12. By utilizing the closed loop system previously described,accurate machining may be performed on the workpiece, for example,within a positioning accuracy of 2 μm.

Workpiece 12 can be translated along the second axis, which isorthogonal to the first axis 75, by a mechanical device, for example,translation table 116. This translation can be performed after apre-determined number of pulses have been fired by the Nd:YAG machininglaser or after a verifying device has determined that the appropriatefeatures have been machined along the first axis 75 for a given locationon the second axis 118. This verifying device is preferably an integralpart of the laser machining system, but can also be a separate device.By scanning the high energy machining beam and the low energypositioning and verifying beam along the first axis 75 and thentranslating the workpiece 12 along the second axis 118 whereafter thescanning along the first axis 75 is repeated, the workpiece 12 can thusbe scanned and machined in a raster-like fashion. The preferredembodiment shown in the drawings incorporates the verifier into themachining system.

Verifying System

The block diagram of FIG. 13 also depicts the operation of oneembodiment of a verifier for the machining system of the presentinvention. When a hole or other feature is machined in workpiece 12,machining beam 125" impinges upon the strip photodetector 146. Machiningbeam, 125, can be composed of a high power pulsed laser, 92, and a lowpower CW laser coaxial with the pulsed laser to provide a continuous lowpower verifying beam. In this preferred embodiment the photodetector islocated beneath the workpiece, but it can be located elsewhere, with thebeam 125" impinging on it through suitable reflective means (not shown).This may be essential if a particular machining operation does notrequire features to be machined through the thickness of the workpiece12.

The output signal pulse 150 of the photodetector strip 146, whichindicates that the beam, 125, has passed through the workpiece, is thenan input to an AND device 148, along with the output 142 of comparator138, which is high when the beam is at the correct feature position. Ifthese two signals 142, 150 correspond, the AND device will have anoutput, verifying that a feature has been machined in a proper location.

Several features will typically be required to be machined along thefirst axis 75 for a given location of the second axis. As the machiningbeam 125" proceeds along the first axis 75, verifying the machining ofeach desired feature as described above, the output of the AND device148 is stored in counter 152. The required number of machiningoperations for this location of the second axis is input from the memory140 into the counter 152. When the actual number of operations is equalto the required number of operations, the counter 152 activates theworkpiece translation controller 154, which sends a signal 156 toactivate the translation table 116, moving the workpiece 12 to the nextposition along the second axis 118.

Other verification methods and apparatus are depicted in FIG. 14.Alternatively, verification of the end point of the desired machiningprocess can be made through other visual adaptive monitoring methodssuch as analysis of the spectra of laser induced plasma from theworkpiece. The workpiece 12 may be made up of different materials 12a,12b which have different spectral lines associated with their plasma. Achange of spectral signature 158 in the plasma or other emissiongenerated by the laser machining (e.g., cutting or drilling) of beam125" or of the low power coaxial laser, would indicate that themachining has proceeded to the interface between different materials.This information, as determined by spectrometer 156 placed proximate theworkpiece 12 and holder 116, could be then fed back to the computer 122.Also, a photocell detector 162 for plume 160 generated by vaporizationof material by the machining process may be placed in the vicinity ofthe workpiece 12. Alternatively, the plume may be detected byobscuration of a reference beam. Determining the presence of a plume 160would indicate that the machining (e.g., drilling) process was not yetcompleted. Determining the absence of a plume would indicate that themachining process has been completed. This information could be then fedback to the computer 112.

In the verifying methods described previously, the same laser firingcircuit card within computer 122 which performs the pulsing of themachining beam could be used to determine the position of the machiningbeam in relation to the part. Simple logic may be used to switchappropriate signals to and from the card.

Design Data Translation System

As part of the maskless positioning system described previously, thepresent invention incorporates a user-friendly input to specify processparameters required for design data preparation. Parameters such aspixel pitch and laser max repetition rates are provided for input. Thisinput is also compatible with the data exchange format (DXF) and tagimage file format (TIFF), recognized as industry standards, tofacilitate input by numerous commercially available computer-aideddesign (CAD) and business graphics application software packages.

The first subsystem for data translation generates pixels for producinggraphic primitives of the desired machining design. Conventional,well-known algorithms are employed to generate pixels corresponding tothe area to be machined. These pixels are typically tailored formicroelectronic design applications, but may also include otherapplications. For example, circles of identical diameter often re-occurthroughout a ceramic package design. These circles may function as pads,vias, and the like, and are required by the design to be identical,except for the location of the center. When pixels for a circle aregenerated, a 45° arc segment is stored in memory. Pixels for anidentically sized circle at another location can be generated by simplyadding fixed offsets to the X-Y coordinates of the 45° arc segment. Thepixels along the remainder of the circumference are obtained by simplynegating and/or transposing the X and Y elements. Line width may also bestored and utilized, as well as smooth corners.

Conventional methods may be used to rotate, translate, reflect,step-and-repeat, magnify and/or shrink desired machining designs.

The second subsystem employed sorts the pixels into pseudo-raster linesby conventional methods. In the case of a TIFF data file, the pixels arepredefined and presorted to a great extent, but use of some portions ofthe two subsystems described herein may still be required. Since pixelsmay be generated so as to be pre-sorted for a given design primitive, aconventional sorting algorithm for such partially stored lists may beemployed. It is desirable to utilize at least partially presorted pixelsto shorten and improve system execution time. Scan lines may also becompressed by combining the pixels of two or more scan lines locatedwithin a predefined interval into one scan line containing both sets ofpixels to increase throughput.

Once the pseudo-raster lines of pixel and scan line information aregenerated, these may be stored in memory 140 of computer 122 andutilized to control the positioning of machining beam 125" on workpiece12 as described above.

The preferred embodiment of the MLDS post processor program begins byaccepting input files in industry standard format. The end-user mayspecify one or more of these, and they may be specified interactively orin a "batch" mode. Input files describe the shapes of the features andtheir locations in the desired design to be machined. After the inputfile is entered, the program computes the pixels location for eachshape. For example, all of the pixels around the circumference of acircle must be computed, given its center and radius. After the pixelsare calculated, they are sorted into scan lines so the machining beamcan expose (machine) the substrate on a line-by-line basis.

The input files do not, in general, present shapes in any predictableorder. Furthermore, the pixels computed for each feature may overlap, interms of scan lines, with pixels from another feature, thusnecessitating the sort step. The final output is a file sorted intolines. Each line is in turn "sub-sorted" into an order that themachining beam can optimally expose. The final output file does notresemble any industry-standard graphics file.

Interference Alignment System

To measure accurately optical performance parameters of scanning system79 and 90, and to align the various components thereof, the presentinvention may utilize in place of or in addition to laser sources 92, 94an alignment laser source for reference and illumination for aninterferometer. Optical parameters measured and corrected in accordancewith the present invention include telecentricity, angle of scannedline, location of the plane in which the scanned line resides,astigmatism, and field curvature.

A preferred embodiment of the interference alignment system is shown inFIG. 15 in connection with the Mid-Objective Scanner describedpreviously, illustrated as machining scanner system 79. This alignmentsystem may be used for tracking scanner 90 in an analogous manner.Alignment laser source 166 is positioned to send object beam 170 alongthe same path as machining beam 25 was described previously. Mountedprior to lens 70 is beam splitter 168 (a partially transmissive,partially reflective mirror mounted at 45° to the path of object beam170) which permits passage of a portion (e.g., 50%) of beam 170'straight through. Continuing object beam 170' then passes through thescanning system components as described, previously where beam 170" isreflected and scanned by galvanometer 50, beam 170"' is then reflectedby concave cylindrical mirror 77 and beam 170"" then becomes focused onoptically flat object mirror 164. Mirror 164 is mounted on translationtable 116 in the same position as workpiece 12 would be located. Mirror164 directs the object beam back through the scanning system, retracingits previous path, until it impinges again upon the beam splitter 168,this time in the opposite direction as before. A portion of thereturning object beam is then reflected 90° downward as beam 170""'. Thebeam portion 180 of original object beam 170 which was not transmittedthrough beam splitter 168 is reflected 90° upward to an optically flatreference mirror 172. Reference mirror 172 reflects reference beam 180back down to beam splitter 168, where a portion 180' is transmittedstraight through so that it interferes coherently with reference beam170""' to form an interference pattern in area 176. The resultinginterference pattern is then evaluated for aberrations throughoutdimensions of the scan. This interference alignment system can be usedfor any scanning system for a working beam, whether it be a high powermachining beam or a low power tracking beam.

Observation of the interference pattern may reveal several potentialproblems. First, there may be "pupil wobble", wherein the interferencepattern physically shifts from side to side, up and down, or acombination of both, as the beam is scanned. Second, there may be achange in the number of fringes that are seen as the beam is scannedalong scan axis 174 or orthogonal translation axis 118 over thedimensions of mirror 164. Third, the total number of fringes may staythe same, but as the beam is scanned the fringes may seem to "collapse"toward the middle of the pattern, or they may seem to "expand" from themiddle of the pattern. Finally, there may be a number of differentpatterns over the length of the scan.

System optical performance parameters can be calculated by counting thenumber of interference fringes that move and appear over the dimensionsof the scan. In the first case discussed above, pupil wobble in one axisrepresents a tilt of the scan line. The wobble in an orthogonal axisrepresents a variation in telecentricity. Angular tilt andtelecentricity can be easily calculated by well known methods in theart. In the second case, by watching for a change in the number offringes and the location in the scan that they occur, actual fieldcurvature can be computed. The value for field curvature (i.e., radiusof curvature) can be calculated by well known techniques by counting thenumber of fringes. In the third case, the tilt of the image plane can bemeasured by well known techniques by the number of fringes that collapseor expand during the scan. In the last case, a variety of aberrationsover the length of the scan can be evaluated by observing fringepatterns and making a conventional analysis.

These effects give accurate, quantifiable measurements of theaforementioned optical parameters of the scanning system. The variouscomponents of the machining and tracking system may be adjusted andaligned during this interferometric scanning process, which gives a"real time" view of image quality while the system is being aligned. Forexample, the tilt of lens 70 or 98 may be adjusted while viewing theinterference pattern generated. The other components of the scanningsystem 79 or 90 may also be adjusted or, if necessary, replaced, toachieve optimum alignment. Since this process uses light to measureimage quality, it has an accuracy on the order of one-quarter of awavelength.

While the present invention has been particularly described, inconjunction with specific preferred embodiments, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. Anapparatus for verifying machining operations comprising:means forgenerating a machining beam; means for generating a tracking beam, saidtracking beam being separate from said machining beam; at least twolenses, one lens being positioned to at least partially focus saidmachining beam, another lens being positioned to at least partiallyfocus said tracking beam; means for tilting each of said at least twolenses at an angle to said machining and tracking beams to add an amountof astigmatism opposite to astigmatism in the remainder of saidapparatus; means for scanning said machining and tracking beams along anaxis after said astigmatism is added by said lenses, said scanning meansbeing movable and having a first surface for reflecting said machiningbeam and a second surface for reflecting said tracking beam; at leasttwo concave cylindrical mirrors having curved reflective surfaces, thecurvatures of the reflective surfaces being substantially identical, oneconcave cylindrical mirror being positioned to receive and interruptsaid reflected machining beam directly from said scanning means firstsurface and at least partially focus said machining beam at a workpiece,another concave cylindrical mirror being positioned to interrupt saidreflected tracking beam directly from said scanning means second surfaceand at least partially focus said tracking beam at a detector; aworkpiece holder for securing a workpiece to receive said machining beamafter reflection from said scanning means and said one concavecylindrical mirror; a detector for receiving said tracking beam afterreflection from said scanning means and said another concave cylindricalmirror; feedback control means for determining the position of thereflected tracking beam relative to said detector and controlling thefiring rate of said machining beam as said machining beam is beingscanned by determining the position of said machining beam relative tosaid workpiece based upon the position of the reflected tracking beamrelative to said detector; a detector located proximate said workpieceholder for detecting the presence or absence of a plume generated byvaporization of material by said machining beam; and electronic controlmeans for verifying the end point of the machining process based uponthe presence or absence of a plume detected by said detector.
 2. Amethod for verifying machining operations comprising the steps of:a)generating a machining beam; b) generating a tracking beam, saidtracking beam being separate from said machining beam; c) providingmeans for scanning said machining and tracking beams along an axis, saidscanning means being movable and having a first surface for reflectingsaid machining beam and a second surface for reflecting said trackingbeam; d) providing at least two tiltable lenses, one lens beingpositioned to at least partially focus said machining beam prior tobeing reflected by said scanning means, another lens being positioned toat least partially focus said tracking beam prior to being reflected bysaid scanning means; e) tilting each of said at least two lenses at anangle to said machining and tracking beams to add to said machining andtracking beams an amount of astigmatism opposite to astigmatism in theremainder of said system; f) providing at least two concave cylindricalmirrors having curved reflective surfaces, the curvatures of thereflective surfaces being substantially identical, one concavecylindrical mirror being positioned to receive and interrupt saidreflected machining beam directly from said scanning means firstsurface, another concave cylindrical mirror being positioned tointerrupt said reflected tracking beam directly from said scanning meanssecond surface; g) simultaneously scanning said machining and trackingbeams with said scanning means; h) reflecting said machining beamreceived directly from said scanning means first surface off said oneconcave cylindrical mirror and at least partially focusing saidmachining beam at a workpiece; i) reflecting said tracking beam receiveddirectly from said scanning means second surface off said anotherconcave cylindrical mirror and at least partially focusing said trackingbeam at a detector; j) securing a workpiece to receive said machiningbeam after reflection from said one concave cylindrical mirror; k)detecting said tracking beam after reflection from said another concavecylindrical mirror; l) determining the position of the reflectedtracking beam; m) controlling said machining beam by determining theposition of said machining beam relative to said workpiece based uponthe detected position of the reflected tracking beam; n) detecting aplume generated by vaporization of material by said machining beam; ando) verifying the end point of the machining process by electroniccontrol means based upon the presence or absence of said plume.